Electrocardiographs (ECGs) play an important role in the detection of many different heart conditions. One of the problems associated with the diagnosis of a heart condition in a patient is that the symptoms indicating the existence of the heart condition may be infrequent, which makes it possible that the heart condition remains undiagnosed for a prolonged period of time, thus exposing the patient to an increased health risk which in extreme cases can lead to the death of the patient.
In order to improve the size of the ECG data set on which the medical practitioner may base his diagnosis, a patient is sometime fitted with a so-called Holter monitor for long-term monitoring of the heart function. A Holter monitor is essentially a portable ECG, which improves the chance that an event is monitored that is indicative of a certain heart condition. However, Holter monitors can cause discomfort to the wearer of the monitor, which is less than ideal from a patient care perspective.
An alternative method to accurately monitor the heart function of a patient was discovered in the 1950s. This method, which uses a so-called ballistocardiograph (BCG), has since gained attention in medical research. A BCG typically comprises one or more sensors mounted in a stationary object in contact with the patient's body, e.g. a bed or a chair that can detect the forces exerted by the patient's body caused by the recoil forces of the blood on the arteries when being pumped out of the (left) ventricle of the heart. This technique has found some useful applications such as in the context of home monitoring, e.g. of chronic patients. This is because a BCG is more convenient for the patient, since the patient no longer has to wear the sensors and/or recording device that are typically associated with a Holter monitor.
An example of such a BCG device is disclosed in U.S. Pat. No. 4,738,264. The device includes a ballistic sensor that may be placed in a bed, and is arranged to monitor the heart rate and the respiration rate of the person sleeping in the bed and to produce an alarm system if combined integrated energy of the combined heart rate and respiration signal during a predefined time window falls under a predefined threshold. This is for instance useful in the early detection of symptoms leading to sudden infant death syndrome (cot or crib death), which is typically preceded by a slowing down of the respiratory and/or heart rate.
However, this prior art device provides data of limited intermediate diagnostic relevance only, because only heart conditions that are characterized by the combination of a fluctuating heart rate and a fluctuating breathing pattern can be detected, e.g. syncope. In contrast, arrhythmias caused by heart conditions such as sick sinus syndrome in which ectopic heart beats may occur or one or more heart beats may be missed do not cause a change in the patient's breathing pattern. Since the breathing component of a BCG dominates the energy content of the BCG signal, it is difficult to detect such arrhythmias.
Moreover, the detection of arrhythmias such as caused by partial or complete AV block, in which the conduction channels, e.g. the AV channel, between the sino-atrial (sinus) node located in the right atrium of the heart and the ventricles are at least partially blocked. Typically, such arrhythmias do not cause a fluctuation in the heart rate such that they cannot be easily detected by the prior art device.
The present invention seeks to provide a method of analyzing a BCG signal that facilitates the detection of at least certain types of arrhythmias in the BCG signal. The present invention further seeks to provide a signal processing device for analyzing a BCG signal that facilitates the detection of at least certain types of arrhythmias in the BCG signal.